Deletion of the activating NK cell receptor NKG2D accelerates rejection of cardiac allografts

It has already been shown that neutralization of the activating NK cell receptor NKG2D in combination with co‐stimulation blockade prolongs graft survival of vascularized transplants. In order to clarify the underlying cellular mechanisms, we transplanted complete MHC‐disparate BALB/c‐derived cardiac grafts into C57BL/6 wildtypes or mice deficient for NKG2D (Klrk1−/−). Although median survival was 8 days for both recipient groups, we detected already at day 5 posttransplantation significantly greater intragraft frequencies of NKp46+ NK cells in Klrk1−/− recipients than in wildtypes. This was followed by a significantly greater infiltration of CD4+, but a lesser infiltration of CD8+ T cell frequencies. Contrary to published observations, co‐stimulation blockade with CTLA4‐Ig resulted in a significant acceleration of cardiac rejection by Klrk1−/− recipients, and this result was confirmed by applying a neutralizing antibody against NKG2D to wildtypes. In both experimental setups, grafts derived from Klrk1−/− recipients were characterized by significantly higher levels of interferon‐γ mRNA, and both CD4+ and CD8+ T cells displayed a greater capacity for degranulation and interferon‐γ production. In summary, our results clearly illustrate that NKG2D expression in the recipient is important for cardiac allograft survival, thus supporting the hypothesis that impairment of NK cells prevents the establishment of graft acceptance.